Vacuum cleaners which utilise cyclonic separators are well known. Cyclonic separators typically comprise a first cyclone stage and a second cyclone stage downstream of the first cyclone stage. The first cyclone stage, which is intended to remove larger dirt and debris, typically comprises a relatively large cyclone chamber, whereas the second cyclone stage, which is intended to remove finer dirt that is able to pass through the first cyclone stage, typically comprises a number of smaller cyclone bodies connected in parallel.
The smaller cyclone bodies are usually arranged in a ring around a longitudinal axis of the cyclonic separator. Through providing a plurality of relatively small cyclones in parallel instead of a single relatively large cyclone, the separation efficiency of the second cyclone stage (i.e. the ability to separate entrained particles from an air flow) can be increased. This is due to an increase in the centrifugal forces generated within the smaller cyclone bodies which cause dust particles to be thrown from the air flow.
Increasing the number of parallel cyclones can further increase the separation efficiency. However, when the cyclone bodies are arranged in a ring this can increase the external diameter of the cyclonic separator, which in turn can undesirably increase the size of the vacuum cleaner. While this size increase can be ameliorated through reducing the size of the individual cyclones, the extent to which the cyclone bodies can be reduced in size is limited. Very small cyclones can become rapidly blocked and can be detrimental to the rate of the air flow through the vacuum cleaner, and thus its cleaning efficiency.
In order to be able to increase the number of cyclone bodies in a cyclonic separator without increasing its external diameter, a recent trend has been to stack the cyclone bodies in two or more layers. Such a configuration is described in GB2475313.
Even when stacked in two or more layers, the cyclone bodies remain connected in parallel. In order that air reaches all layers of cyclones, a system of conduits, or ducts, is provided within the second cyclone stage. By way of example, FIG. 1 shows a schematic representation of stacked cyclone bodies 10 according to a known configuration which shows sets of conduits 16 and 18 that convey air to the cyclone bodies 10, and a further set of conduits 20 that convey air from the cyclone bodies 10. Two layers of cyclone bodies are provided, a lower layer L and an upper layer U, each cyclone body 10 comprising an inlet 12 and a vortex finder 14 that serves as an outlet. Sets of inlet conduits 16 and 18 are provided to convey air from the first cyclone stage to the cyclone bodies 10 of the second cyclone stage. One set of these inlet conduits 16 is configured to convey air to the cyclone bodies 10 on the lower layer L, and the other set of conduits 18 is configured to convey air to the cyclone bodies 10 on the upper layer U. The vortex finders 14 of each of the cyclone bodies 10 on both the upper and lower layers L, U then feed into one of a number of outlet conduits 20 which convey the cleaned air downstream to a next stage in the cyclonic separator.
As can be seen from FIG. 1, the fluid paths through the conduits are different. This can lead to uneven loading of the air supply on the cyclone bodies 10. For example, some cyclone bodies, through which an easier fluid path is available, will be under a greater load than other cyclone bodies that provide a more tortuous fluid path for the air to take. This creates inefficiency within the cyclonic separator, and can reduce the overall efficiency of the vacuum cleaner.